Recently, with the advance in making video sound high-fidelity and with the change of video sound from monaural to stereo or bilingual, the frequency band-width of audio signals has been expanded and high-quality transmission system has been required.
To satisfy such a requirement, there has been proposed an audio recording system of a video tape recorder (VTR) in which an audio signal is recorded in the form of an FM audio signal through a helical scanning recording system using a plurality of write heads in the same manner as video signal recording, besides a conventional audio recording system in which an audio signal is recorded simply continuously in an audio track in parallel to the running direction of a tape.
For example, the proposed recording system is mainly classified into two types, namely, a VHS type and a beta-format or 8 mm video type. The VHS type recording system has audio-exclusive heads and video-exclusive heads so that an FM audio signal recorded in a deep layer portion of magnetic recording layers of a tape by use of the audio-exclusive heads and then a video signal is recorded in a shallow layer portion of the tape by use of the video-exclusive heads. The beta-format or 8 mm video type recording system has video heads by which a video signal and an FM audio signal are recorded at once in the form of a composite signal.
In either systems, as shown in FIG. 6, an FM audio signal is recorded by use of two read-write heads 2 and 2' on a tape 3 running in the direction of the arrow A while the tape 3 is wound on a rotary drum 1 helically over a half round thereof, the rotary drum 1 rotating in the direction of the arrow B. The read-write heads 2 and 2' are provided on the rotary drum 1 in positions separated by 180.degree. from each other and being symmetrical with respect to the rotational axis of the rotary drum 1. The heads 2 and 2' shown in FIG. 6 are heads related to audio, that is to say, video-exclusive heads are not shown even in the case such video-exclusive heads are provided.
FIG. 7 is a view showing visually signals recorded on the tape 3, seen from the base surface of the tape.
In the drawing, an audio track 4 records thereon an audio signal simply and continuously, and a control track 5 controls tracking of the rotary heads.
An FM audio signal is divided into parts each equivalent to one field (1/60 seconds in the case of NTSC) of video signal. The parts are recorded, by use of the heads 2 and 2', on FM audio tracks 6 and 6' obliquely relative to the running direction of the tape 3. The FM audio tracks 6 and 6' adjacent to each other are constructed so that a backward end portion a of a preceding track and a forward end portion b of a succeeding track overlap each other by about 5%.
FIG. 8 is a block diagram showing an example of a conventional apparatus for reproducing a continuous audio signal from such an FM audio signal recorded as described above.
The heads 2 and 2' are respectively connected in series to pre-amplifiers 7 and 7' to form two channels CH1 and CH2. The output signals of the pre-amplifiers 7 and 7' are respectively fed to a changeover switching circuit 8, for example, constituted by an analog multiplexer.
Assuming now tha the head 2 of the channel CH1 puts out an FM regenerative signal, then the FM regenerative signal is amplified by the pre-amplifier 7 and fed to the changeover switching circuit 8.
The changeover switching circuit 8 is constructed to switch over to a signal regenerative channel (hereinafter referred to as "live channel"). Accordingly, the signal of the channel CH1 is passed through the changeover switching circuit 8. Then, unnecessary part of the signal such as a video signal mixed therein or a noise is cut off by a band-pass filter (hereinafter referred merely to "BPF") 9. Then, only an FM audio signal is demodulated by the demodulator 10 so that an audio signal is outputted.
When the head 2' of the other channel CH2 then outputs an FM regenerative signal, on the contrary, the changeover switching circuit 8 is operated to switch over to the channel CH2. (Hereinafter, the channel having no signal output is referred to as "dead channel", and the dead channel starting signal output is referred to as "following channel".)
The aforementioned procedures are repeated alternately, so that audio signals thus demodulated are put out in the form of a continuous signal.
Although the relative positions (of symmetry) of the heads 2 and 2' on the rotary drum as shown in FIG. 6 may be produced with high accuracy, it is impossible to avoid a little phase difference of FM regenerative signal caused by factors, such as: a difference between the head positions at recording time and at reproducing time caused by a slight manufacturing tolerance; expansion and contraction of the tape caused by changes in temperature, in humidity, and in time aging; expansion and contraction of the rotary drum; a difference in lagging between the circuits; and the like.
Therefore, a phase difference occurs between the FM regenerative signals of the channels CH1 and CH2 as shown in FIG. 9, so that a discontinuous portion is produced in the waveform of the FM audio signal by the switching operation of the changeover switching circuit 8.
In FIG. 9, the diagrams (a) and (b) show waveforms of FM audio signals outputted from the channels CH1 and CH2, respectively. By switching the changeover switching circuit 8 in an overlap period as shown in the diagram (c), the waveform of a continuous FM audio signal as shown in the diagram (d) can be obtained.
However, slight phase differences in the waveform of the signals shown in the diagrams (a) and (b) of FIG. 9 are caused by jitter. If the time axis in the vicinity of the switching points S as related to the waveform of the signal shown in the diagram (d) is enlarged, a discontinuous portion of the waveform of the signal caused by the switching can be observed as shown in the diagram (e).
In the rear of the discontinuous portion, the fequency is shifted by a period determined by the bandwidth of the BPF 9.
Accordingly, if the signal is demodulated into an audio signal by the demodulator 10, disorder D is produced in the waveform of the audio signal as shown in the diagram (f) of FIG. 9.
To prevent the disorder of the waveform of the audio signal, a measure of temporarily storing the level of the waveform of the demodulated audio signal or temporarily holding the linear differential value thereof as shown in the diagram (g) of FIG. 9 before the procedure is shifted to the demodulated signal of the channel CH2 has been proposed. However, the problem in discontinuity of the waveform could not be solved prefectly.
Whenever the respective head is changed over, discontinuity occurs in a predetermined period (60 times per second in the case of NTSC). In the case of complex-waveform sounds such as a percussive sound, an orchestra sound and a consonant sound, the discontinuity becomes inconspicuous. However, in the case where simple-waveform sounds near to pure sounds such as a wind sound, an organ sound and a vowel sound (particularly, "a" and "o") are continued, the continuity becomes conspicuous as noise.